Perianth evolution in the sandalwood order Santalales.

Flowers of Santalales remain largely unexplored with several questions of homology unanswered despite the large size of the order. Morphological and ontogenetic floral studies have the potential to identify new informative characters. We studied floral development in species of Loranthaceae, Santalaceae, Opiliaceae, and "Olacaceae" with scanning electron microscopy to clarify the origin and evolution of the perianth in Santalales. The perianth is either dichlamydeous or when monochlamydeous interpreted as sepals, petals, or tepals. A girdling calyculus of debatable origin is found in some clades. We show that species of Diogoa and Heisteria in "Olacaceae" have a dichlamydeous perianth, unlike Olax in which the calyx is replaced by a calyculus. The calyculus arises by development of two lateral primordia, supporting the hypothesis of bracteole origin. A calyculus with similar development is present in species of Loranthaceae and possibly of Opiliaceae, suggesting a position of Olax closer to these families than to traditional genera of "Olacaceae". The monochlamydeous perianth in Santalaceae is shown to correspond to petals of other members of Santalales. Flower ontogenetic evidence suggests a repeated loss of the calyx, replacement by a calyculus, and further loss, leading to monochlamydeous perianths in Santalaceae.